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Homework Help: Organ pipe sound source

  1. Nov 28, 2017 #1
    1. The problem statement, all variables and given/known data
    Organ pipe sound is driven by a compressed air jet flicking alternately in and out of the pipe in step with the fundamental frequency. My question is whether sound waves of the higher frequency harmonic series are emitted during the intervals when the jet is outside of the pipe and not contributing energy to the system, and if so, what drives them?

    Thank you.

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Nov 28, 2017 #2

    haruspex

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    What determines the fundamental frequency? Think about why the jet should flick in and out at that frequency.
     
  4. Dec 4, 2017 #3

    PeterO

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    I think you are forgetting how the Organ pipe sounds. Remember resonance. The air can flick in and out at any rate at all, but you only get resonance with specific frequencies (fundamental and overtones), so you only hear specific frequencies sounding.
     
  5. Dec 4, 2017 #4

    haruspex

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    Not sure how you mean that. As I hinted, it flicks in and out at the rate it does for a reason. Understanding that reason is key to answering the question.
     
  6. Jan 16, 2018 #5
    The question is the energy source to drive the harmonic frequencies. If the jet flicks in and out of the organ pipe, adding energy only while flicking into the pipe, and if the fundamental frequency of the pipe were 110 Hz, then during each second there are one hundred and ten 4.5 ms gaps during which energy is not being added to the pipe. But during those 4.5 ms gaps, harmonic sound waves are being generated. For example, to sustain the harmonic frequency of 1100 Hz requires an energy pulse inside the pipe every 0.9 ms. Where is that energy coming from during the aforementioned gaps?
     
  7. Jan 16, 2018 #6

    haruspex

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    Please try to answer my question in post #2, even though you do not yet see the relevance.
     
  8. Jan 17, 2018 #7
    The fundamental frequency imposed on the jet is determined by the length of the pipe and the speed of sound, that is the time that it takes for each compression/rarefaction event initiated by the jet to transit the length of the pipe.
     
  9. Jan 17, 2018 #8

    haruspex

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    No, the jet does not initiate these events by itself.
    The air oscillates within the pipe. The rebound pushes the jet out. That is how the length of pipe dictates the flick rate. So the air in the pipe is a reservoir of energy.
    Also, remember that the sound consists of a sequence of pressure pulses. The gaps in between are all part of the sound. But there is no gap in the energy. At any time, at a given point, the air is at a different pressure from ambient, or is moving, or a combination of the two.
     
  10. Jan 17, 2018 #9
    Many thanks!!
     
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